Question

The value of one barn in SI unit is

• A. \(10^{-28} m^2\)
• B. \(10^{-20} m^2\)
• C. \(10^{-16} m^2\)
• D. \(10^{-32} m^2\)
• E. \(10^{-15} m^2\)

Hint:

1 barn = 100 fm\(^2\) (femtometer squared), and 1 fm = \(10^{-15}\) m.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
The 'barn' is a unit of area used in nuclear and particle physics to quantify the cross-section of a scattering process. The question asks for its value in the SI base unit for area, which is the square meter (m\(^2\)).
Step 2: Detailed Explanation:
The size of an atomic nucleus is on the order of femtometers (1 fm = 10\(^{-15}\) m). The cross-sectional area of a typical nucleus is therefore on the order of (10\(^{-14}\) m)\(^2\) = 10\(^{-28}\) m\(^2\). This value was defined as one barn.
The etymology of the name comes from the phrase "as big as a barn," which was wartime slang used by physicists at Purdue University to describe the large probability of interaction for certain nuclear reactions.
The exact definition is: \[ 1 \text{ barn} = 10^{-28} \text{ m}^2 \] This can also be expressed in other units: \[ 1 \text{ barn} = 100 \text{ fm}^2 = 10^{-24} \text{ cm}^2 \] Step 3: Final Answer:
The value of one barn in SI units is 10\(^{-28}\) m\(^2\). This corresponds to option (A).